(a) Field of the Invention
This invention relates to novel designs of an axial flow fan, more specifically novel designs of the casing and hub of an axial flow fan. The casing and hub of an axial flow fan according to this invention are provided with cross-sectional inside surface configurations in the form of a circle or an ellipse and at leat one projection is provided in the air flow passageway inside the casing. The projections are provided on the hub or on the casing in order to break up efficiency-reducing large air swirls generally produced in the areas around the outlet port of the casing and at the rear end of the hub. It is also possible to provide projections both on the hub and casing.
(b) Prior Arts
Conventionally, an axial flow fan is provided with a casing enclosing a fan wheel having a diameter which first decreases in the air flow direction from the inlet port to a constricted "necked-down" portion, and increases again from this necked-down portion toward the outlet port as shown in FIG. 2, such as disclosed in U.S. Pat. No. 4,221,546. Another type of prior art axial fan has a cylindrical configuration in the axial central plane of the casing as shown in FIG. 3. In both of the above cases, there are no projections provided in the air flow passageway at the area around the outlet port. Because of this feature, the air flows through the air duct of the casing having a rather complicated inside surface configuration. Further, large air swirls are generated in the area around the outlet port because the cylindrical shape which serves to guide the air flow abruptly ends there. Still further, large swirls of air are also generated at the rear end of the hub because the cylindrical shape abruptly ends there. The abovementioned complicated air flows and swirls reduce the efficiency of the axial flow fans.